19 
54¢ For a point Q on the sea-bed 
in fige7, the theory of complete 
reflection gives a reflected 
pulse fram E'' exactly equal and 
synchronous with the incident 
pulse from E and the resultant 
pressure/time curve is simply the 
original pulse doubled in 
pressure. In practice, such 
doubling is not attained but the 
maximum pressure, for example, at 
a point on a chalk sea-bed, may be 
50% more than for a point in mid- 
water at the same distance from a 
similar charge. 
PRESSURE 
55- Summing up, for a charge at 
some distance from the sea-bed, 
the effect of the sea-bed is 
positive in character leading to 
additional pressure which is 
largest in general at points on 
Fig.8 - Pressure/time variation at the sea-bed. An upper limit to 
a@ point near the sea-bed such a sea-bed effect’ can be 
obtained by assuming complete 
reflection to occur, the pressure in the reflected pulse then corresponding 
to that from a charge in open water at the image B'' in fig. 7. Asa 
final point, there is some limited experimental evidence that the 
reflection becomes more complete at more distant points to the side 
corresponding to near glancing incidence. 
Charge on sea-bed 
56. When the charge is on or near the sea-bed the events in the inner 
region, where the pulse is "born", are themselves affected by the 
presence of the sea-bed and the direct pulse sent out from the charge is 
no longer the same as for a similar charge well away from the sea-bed. 
57. For the theoretical case of an infinitely rigid sea-bed it would 
still seem permissible, however, to, assume that the presence of the sea-bed 
is equivalent to an equal charge at the image point, the new effect being 
that the charge and image charge interfere with one another. For the 
limiting case of a hemispherical charge of weight W on the sea-bed, 
theory would predict the same effects as a spherical charge of weight 2 W 
in mid-water. Equations 11 and 16 would then indicate a resultant pulse 
from the charge on the qea-bed having both pressure and time scale 
increased by a factor 2 = 1.26 as compared with the same charge in 
open water. The corresponding energy per unit area of pulse front 
would, by virtue of equation 13, be simply doubled. This does not, of 
course, mean any change of total energy in the pulse since this is 
propagated out through a hemisphere when the charge is on the sea~bed and 
not through a sphere round the charge as in open water far from the 
sea-bed. These theoretical results for an infinitely hard sea-bed are 
subject to modification for actual sea=beds. 
58. In practice, soft mud sea-beds behave effectively as further water, 
that is, the pulse is unaffected by the presence of the sea-bed and is 
the same as for a charge at the same distance in nid-water. 
